The Adventures of Tom Sawyer by Mark Twain Chapter 10 Page 7

“Hold on! Don’t do that. A pin’s brass. It might have verdigrease on it.”

“What’s verdigrease?”

“It’s p’ison. That’s what it is. You just swaller some of it once — you’ll see.”

So Tom unwound the thread from one of his needles, and each boy pricked the ball of his thumb and squeezed out a drop of blood. In time, after many squeezes, Tom managed to sign his initials, using the ball of his little finger for a pen. Then he showed Huckleberry how to make an H and an F, and the oath was complete. They buried the shingle close to the wall, with some dismal ceremonies and incantations, and the fetters that bound their tongues were considered to be locked and the key thrown away.